This invention relates to gas cylinder filling methods, and more particularly to filling vessels with gas mixtures by pressure measurement techniques with gas mixtures comprising a major component and one or more minor components wherein the major component has a lesser density than at least one of the minor components.
Users of gas mixture products often have a need for uniform mixtures of gases at precise concentrations. However, because of certain conventional gas cylinder filling techniques, gas mixtures sold in containers such as gas cylinders are not always well mixed and do not always contain the desired concentration of each component. Generally, when two or more gases are charged into a gas cylinder, stratification of the gases in the cylinder will exist until the gas mixture reaches equilibrium. In the case of high pressure gas mixtures, this can take long periods of time, on the order of several days or more, unless special measures to mix the cylinder contents are undertaken.
The problem of stratification of components of a gas mixture in a newly filled cylinder can be pronounced when a gas mixture comprising a very small amount, for example about 1%, of one gas and about 99% of a second gas (by volume) is charged into a gas cylinder. When filling gas cylinders with mixtures of gases by pressure filling procedures it is customary to introduce the minor component into the cylinder first using a low pressure gauge to ensure a more precise transfer of the desired quantity of the minor component. Since precision pressure gauge readings are usually accurate to within about 0.1% of full scale, the error will be small when this procedure is used. If the minor component is lighter than the major component, relatively rapid mixing of the gases will occur. However, if the minor component has a greater density than the major component and the minor component is charged to the cylinder first, it will take considerably longer for the gas mixture to reach uniform concentration throughout the cylinder because gravitational forces impede upward movement of the more dense gas. Consequently, when the gas user requires a homogeneous gas mixture, the cylinder must be stored sufficiently long for substantially complete mixing to occur before the cylinder can be put into service, or measures taken to effect homogenization of the gas mixture.
Uniform mixing during filling of gas cylinders containing gas mixtures is also important because it minimizes temperature variation throughout the gas mixture, which in turn ensures more accurate measurement by pressure of the quantity of each component of the gas mixture introduced into the cylinder.
Various efforts have been undertaken to effect a more rapid mixing of gas mixtures in cylinders. One technique is to introduce the lighter gas component into the bottom part of the cylinder by means of a mixing tube. This will force the light gas to migrate upwardly through the heavy gas, thereby causing the gases to mix. Another technique is to roll the cylinders until the contents are uniformly mixed. Each of these procedures requires considerable handling of the cylinders, which increases the cost of filling the cylinders with gas mixtures.
Because of the importance of attaining immediate homogeneity of vessel-contained gas mixtures, methods of effecting more rapid mixing in vessels of gas mixtures comprising a minor component that has a greater density than the major component are continuously sought. The present invention provides such a method.